The movement of a read/write head of a disk drive from a present position to a desired position, where the head can read data from or write data to information bearing tracks thereof, is referred to as a “seek operation” between tracks. Preferably, the seek operation takes as little time as possible, consistent with minimum final position error and settle time prior to entering the track following phase in which data may be read or written. Track following relies on the head reading position reference information, frequently referred to as “servo burst patterns”, on at least one disk surface. Such position reference information may either be on a dedicated servo surface and read by a dedicated servo head or it may be dispersed (i.e., embedded) as servo sectors on the data surface and read by the data head. Generally, the position reference information does not give an absolute position of the head but only an offset position relative to a single track or within a small group of tracks. The seek operation conventionally uses this information to update a register containing either the absolute position or the number of tracks to go to the desired track.
A seek operation typically includes an “acceleration phase” during which the head is accelerated (e.g., in an open loop fashion) toward the desired position (e.g., a desired track), followed by a “deceleration phase” in which the head is decelerated (e.g., under some sort of closed loop control) to come to rest approximately on the desired track. There may also be an intermediate “coasting phase” between the acceleration and deceleration phases. Additionally, following the deceleration phase there is typically a “settle phase” and an “on-track phase” that are used for finer position adjustment of the read/write head.
During the deceleration phase, many disk drives use what is known as a Proximate Time Optimal Servo (PTOS) deceleration control algorithm. With a PTOS algorithm, in order to move a read/write head from a present position (e.g., a present track) to a desired position (e.g., a desired track), a single deceleration profile (in phase space) is determined. Then, a linear velocity controller is used to try to hold the head (through control of an actuator assembly including a voice coil motor) to that predetermined single deceleration profile. As the head deviates from the predetermined single profile, the linear velocity controller drives the head higher or lower in phase space to get it back to that profile. More specifically, using a PTOS type algorithm a servo system determines the head's present velocity and present position (e.g., the remaining distance to the desired track) at each sample time or track crossing, and compares the present velocity/position with the predetermined single deceleration profile. Based on this comparison, the servo system appropriately increases or decreases the drive current to the voice coil motor (VCM) if the velocity is above or below the velocity given by the predetermined deceleration profile, in an attempt to follow the predetermined profile. This will now be explained with reference to the phase space diagram of FIG. 1.
FIG. 1 is a phase space diagram, with the axis labeled x representing position of a read/write head, and the axis labeled v representing velocity of the head. In the diagram, the origin 110 represents zero velocity and zero distance from the desired position, and a point in space 112 represents the present velocity and present position of the head. The present position can be, e.g., in terms of the number of tracks (e.g., 3500 tracks) from the desired track. In FIG. 1, the solid line 114 represents an exemplary single deceleration profile that was determined using a PTOS type algorithm. The dashed line 116 represents the exemplary movement of the head, in phase space, as it moves from the present velocity/position 112 to the desired velocity/position 110. Notice how the dashed line oscillates around the desired trajectory 114 in an aim to get the head back to the desired trajectory. It would be beneficial to reduce and preferably eliminate the inefficiencies associated with trying to drive the head higher or lower in phase space to get it back to the desired trajectory.